High strength, high stress corrosion cracking resistant and castable al-zn-mg-cu-zr alloy for shape cast products

ABSTRACT

The present invention provides an Al—Zn—Mg—Cu casting alloy that provides high strength for automotive and aerospace applications and optimized stress corrosion cracking resistance in highly corrosive and tensile environments. The inventive alloy composition includes about 3.5 wt. % to about 5.5 wt. % Zn; about 1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; and a balance of Al and incidental impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/449,273, filed Apr. 17, 2012, which is a continuation of U.S.application Ser. No. 11/856,631, filed Sep. 17, 2007, which claims thebenefit of U.S. Provisional Application No. 60/826,131, filed Sep. 19,2006, each application of which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

This invention relates generally to aluminum alloys for aerospace andautomotive shaped castings having high tensile strength and highresistance to stress corrosion cracking (SCC).

BACKGROUND OF THE INVENTION

Cast aluminum parts are used in structural applications in automobilesuspensions to reduce weight. The most commonly used group of alloys,Al7SiMg, has well established strength limits. In order to obtainlighter weight parts, higher strength material is needed withestablished material properties for design. At present, cast materialsmade of A356.0, the most commonly used Al7SiMg alloy, can reliablyguarantee ultimate tensile strength of 290 MPa (42 ksi). and tensileyield strength of 220 MPa (32 ksi) with elongations of 8% or greater.

In applications where high strength is required forged products aretypically used. Forged products are disadvantageously more expensivethan cast products. Considerable cost savings may be realized in bothautomotive and aerospace applications if cast products can be used toreplace forged products with little or no loss of strength. elongationperformance, general corrosion resistance. stress crack corrosionresistance and fatigue strength.

A variety of alternative casting alloys exist that exhibit higherstrengths than Al7SiMg alloys. However these exhibit problems incastability, corrosion performance or fluidity, which are not readilyovercome. For example, U.S. patent application Ser. No. 11/111,212,titled “Heat Treatable Al—Zn—Mg—Cu Alloy for Aerospace and AutomotiveCastings”, filed on Apr. 21, 2005, discloses an Al—Zn—Mg—Cu alloy forshaped castings having high fatigue resistance and high strength that issuitable for automotive and aerospace applications. While theAl—Zn—Mg—Cu alloy disclosed in U.S. patent application Ser. No.11/111,212 provides good general corrosion resistance, it has beendetermined that the alloy exhibits a less than optimum stress corrosioncracking (SCC) resistance in environments of high tensile stress andcorrosion, which could limit it's application.

In light of the above, a need exists for a casting alloy havingstrengths suitable for high strength automotive and aerospaceapplications, while simultaneously providing high stress corrosioncracking (SCC) resistance. It is further desired that the alloy maintainacceptable levels of fatigue resistance and general corrosion resistanceand castability to be suitable for providing shaped castings foraerospace and automotive applications.

SUMMARY OF THE INVENTION

Generally speaking, the present invention provides an Al—Zn—Mg—Cu alloyfor shaped castings having ultimate tensile strengths greater than thatachieved by comparable castings of A356, while maintaining corrosionperformance suitable for automotive and aerospace applications,specifically including a good resistance to stress corrosion cracking(SCC) in severely corrosive environments of high tensile stress.Broadly, the inventive alloy is composed of

about 3.5-5.5 wt. % Zn,

about 1.0-3.0 wt. % Mg,

about 0.5-1.2 wt. % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

a balance of Al and incidental impurities.

In one aspect of the present invention, the stress corrosion cracking(SCC) resistance of the alloy was increased by optimizing the amount ofZn and Mg, as well as the amount of Cu, Specifically, to achieveoptimized stress corrosion cracking (SCC) performance the Mg and Zncontent is to he limited to less than or equal to 6.0 wt. %, and the Cucontent is incorporated in greater than or equal to 0.5 wt. %. Thecorrosion character of the Al—Zn—Mg—Cu alloy of the present inventionwhen in an overaged condition exhibits pitting corrosion, which is thepreferred mode of corrosion in comparison to intergranular corrosion.

In another aspect of the present invention, a method of manufacturing ashaped casting is provided in which an Al—Zn—Mg—Cu alloy providesstrengths greater than that achieved by comparable castings of A356,while maintaining corrosion performance that is suitable for automotiveand aerospace applications, specifically including a good resistance tostress corrosion cracking in severely corrosive environments of hightensile stress. Broadly, the method includes the steps of:

preparing a molten mass of an aluminum alloy composed of:

about 3.5-5.5 wt. % Zn,

about 1-3 wt. % Mg,

about 0.5-1.2 wt % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

incidental impurities;

casting the melt into a cast body; and

heat treating the cast body to an overaged temper;

casting at least a portion of the melt into a mold to provide a shapedcasting; and

heat treating the shaped casting to an overaged condition.

In one embodiment, the optimum conditions for the alloy to achieve highstress corrosion cracking (SCC) resistance and high tensile strengthincludes an alloy composed of a Mg and Zn content of 6.0 wt. % or lessand a Cu content greater than 0.5 wt. % in combination with casting andheat treating to an overaged temper. In the overaged condition, theinventive Al—Zn—Mg—Cu alloy exhibits only pitting corrosion mode(general corrosion), which is the preferred corrosion mode in comparisonto intergranular corrosion (IG) when tested under ASTM G110 conditions.

The inventive Al—Zn—Mg—Cu alloy and method of producing a shaped castingin addition to providing levels of strength and corrosion performancethat were previously not obtainable with prior Al—Zn—Mg—Cu alloys,additionally provides acceptable hot cracking performance and fluidityfor casting shaped products.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, wherein like referencenumerals denote like elements and parts, in which:

FIG. 1 is a plot illustrating the effect of Cu, Mg, and Zn on the stresscorrosion cracking (SCC) resistance of an Al—Zn—Mg—Cu alloy inaccordance with the present invention when subjected to a 7 day boilingsalt SCC test (ASTM G103) under a tensile stress of 240 MPa.

FIG. 2 is a plot illustrating the effect of Cu, Mg, and Zn on the StressCorrosion Cracking (SCC) resistance of an Al—Zn—Mg—Cu alloy inaccordance with the present invention when subjected to a 7 day boilingsalt test (ASTM G 103) under a tensile stress of 160 MPa.

FIG. 3 is a plot illustrating the effect of Cu on the tensile yieldstrength of an Al—Zn—Mg—Cu alloy in accordance with the presentinvention.

FIGS. 4 a-4 c are photographs of test specimens formed in accordancewith the present invention, and comparative examples, evaluated forgeneral corrosion performance by immersion in a NaCl+H₂O₂ solution inaccordance with ASTM G110.

FIGS. 5 a-5 c depict graphs of the depth of corrosion in microns of testspecimen evaluated by ASTM G110 corrosion testing, wherein the testspecimen include Al—Zn—Mg—Cu compositions including greater than 0.5 wt.% Cu and comparative examples having less than 0.5 wt. % Cu.

FIGS. 6 a-6 c are photographs of the side cross section of testspecimens formed in accordance with the present invention, andcomparative examples, illustrating the degree of intergranular corrosiveattack.

FIG. 7 is a graph of the hot cracking index verses the Cu content ofpencil probe castings of Al—Zn—Mg—Cu alloys, in accordance with thepresent invention.

FIG. 8 is a graph of the fluidity verses the Cu content of spiral moldcastings of Al—Zn—Mg—Cu alloys, in accordance with the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides an Al—Zn—Mg—Cu alloy having yieldstrengths, fatigue strength, general corrosion performance, and stresscorrosion cracking (SCC) performance suitable for automotive andaerospace applications. while maintaining castability. All componentpercentages herein are by weight percent unless otherwise indicated.When referring to any numerical range of values, such ranges areunderstood to include each and every number and/or fraction between thestated range minimum and maximum. A range of about 0.5-1.2 wt. % Cu, forexample, would expressly include all intermediate values of about 0.6,0.7, 0.8 and all the way up to and including 1.1 wt. % Cu. As usedherein, the term “incidental impurities” refers to elements that are notpurposeful additions to the alloy, but that due to impurities and/orleaching from contact with manufacturing equipment. trace quantities ofsuch elements being no greater than 0.05 wt. % that may, nevertheless,find their way into the final alloy casting or casting alloy ingot.

The Al—Zn—Mg—Cu casting alloy of the present invention is composed of:

3.5-5.5 wt. % Zn,

about 1.0-3.0 wt. % Mg,

about 0.5-1.2 wt. % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

a balance of Al and other incidental impurities.

In one aspect of the present invention, the Mg, Zn and Cu content of thealloy is selected to provide increased strength and stress corrosioncracking resistance. The stress corrosion cracking performance of thealloy is effected by both chemical and physical factors. From a physicalstandpoint, to provide an alloy having good stress corrosion crackingperformance the degree of precipitation within the alloy must be of alevel to provide sufficient strength, but not be so great as to causeembrittlement of the alloy. From a chemical standpoint, resistance tostress corrosion cracking requires resistance to chemical attack bycorrosion.

Preferably, the Al—Zn—Mg—Cu alloy provides increased stress crackingcorrosion (SCC) resistance with a composition of alloying elements andratios including a Mg and Zn content of about 6.0 wt. % or less, and aCu content greater than 0.5 wt. %, preferably ranging from 0.5 wt. % to1.2 wt. %, so long as the Cu content is not so great to causeembrittlement of the alloy. Specifically, in one preferred embodiment,high tensile yield strengths on the order of about 300 MPa and StressCorrosion Cracking resistance at a stress of up to 240 MPa may beprovided by the Al—Zn—Mg—Cu alloy, in accordance with the presentinvention.

In one aspect of the present invention, the Mg and Zn content of theAl—Zn—Mg—Cu alloy is preferably selected to provide strength enhancingMgZn₂ precipitates. Preferably, the Zn/Mg ration is about 3.3 or lessand the total of the Mg and Zn content is less than about 6.0 wt. % ofthe alloy composition. During processing of the Al—Zn—Mg—Cu alloy of thepresent invention, Mg and Zn from the alloy precipitate to form MgZn₂ inthe Al Matrix and at the grain boundary. Increasing the Mg and Zncontent to greater than 6.0 wt. % may produce excess MgZn₂ at the grainboundaries, resulting in reduced stress corrosion cracking (SCC)resistance. In one preferred embodiment, the Zn is at a concentration ofabout 3.8 to 4.6 wt. %, most preferably ranging from 4.0 wt. % to 4.4wt. %. In one preferred embodiment, the Mg is at a concentration ofconcentration of about 1.2 wt. % to 1.8 wt. %, most preferably rangingfrom 1.4 wt. % to 1.6 wt. %.

In another aspect of the present invention, the Cu content of theAl—Zn—Mg—Cu alloy is select to substantially reduce intergranularcorrosion, while further providing precipitate hardening mechanismsthrough the formation of Al₂Cu (θ-phase) precipitates or Al₂CuMg(s-phase) precipitates. Preferably, the Cu content is selected toincrease the corrosion potential of the precipitate free zone of thealloy relative to the alloy's aluminum matrix. In one embodiment, the Cucontent is greater than about 0.5 wt. %, preferably ranging from about0.5 wt. % to about 1.2 wt. %, even more preferably ranging from about0.65 wt. % to about 1.0 wt. % and most preferably ranging from about 0.7wt. % to 0.8 wt. %. Increasing the Cu to greater than about 1.2 wt. %may result in an excess of constituent particles of Al—Fe—Cu at thegrain boundary, which may decrease the alloy's ductility, fatigueresistance and toughness.

The Al—Zn—Mg—Cu alloy of the present invention provides increased stresscorrosion cracking (SCC) resistance, wherein in one aspect the Mg, Znand Cu constituents are selected to substantially reduce intergranularcorrosion. As opposed to general corrosion or pitting, intergranularcorrosion can be particularly troublesome as occurring below the surfaceof the casting, wherein severe corrosion may occur without anyindication by visual inspection. Additionally, localized corrosion atthe grain boundaries by intergranular corrosion accelerates failure, asopposed to the more homogenous corrosion provided by pitting.

Intergranular corrosion in prior Al—Zn—Mg alloys results fromdifferences in the corrosion potential between the Aluminum AlloyMatrix, the precipitate free Zone (PFZ) and the grain boundary of thecasting alloy. The differences in corrosion potential in prior alloysresults from the incorporation of the elements that are at least in partintroduced for precipitate hardening mechanisms, such as Mg and Zn.

Precipitation at the grain boundary after quench is initiallycontinuous, since diffusion at the grain boundary is very fast relativeto diffusion within the Al Matrix due to the grain boundary's openstructure. Therefore, precipitates such as MgZn₂ more readily form largecontinuous precipitates at the grain boundary, whereas the Al matrixrestricts diffusion and growth of precipitates resulting in a moreuniform fine dispersion of precipitates. At the interface of the Almatrix and the grain boundaries is the precipitate free zone (PFZ),being substantially free of precipitates, in which the alloyingelements, such as Mg and Zn, are in solution.

In one instance, intergranular corrosion results from the equivalent ofa galvanic cell (micro-galvanic corrosion) formed between the AluminumMatrix and the precipitate free zone (PFZ) due to the potentialdifference between the differing composition of the aluminum matrix andthe composition of the precipitate free zone (PFZ). In prior Al—Zn—Mgalloys the corrosion potential difference between the matrix and theprecipitate free zone (PFZ) can be significant, in which the casting isparticularly susceptible to intergranular corrosion and typicallydegrades. Such degradation can results in decreased resistance to stresscorrosion cracking and premature failure of the casting. In priorAl—Zn—Mg alloys the corrosion potential difference between the AluminumMatrix and the precipitate free zone (PFZ) typically results fromelements of Mg and Zn incorporated into the PFZ solution, wherein theincorporation of Mg and Zn decreases the corrosion potential of theprecipitate free zone (PFZ) relative to the Al matrix.

In one aspect of the present invention the Cu content is selected toincrease the corrosion potential of the precipitate free zone (PFZ)relative the Al matrix. Preferably, the incorporation of Cu into thealloy, and hence the precipitate free zone, offsets the decrease incorrosion potential resulting from the Mg and Zn incorporated in themetal solution at the precipitate free zone, preferably to provideuniformity in corrosion potential between the precipitate free zone andthe aluminum matrix. The alloy of the present invention by specifyingand controlling the alloying amounts maintains a balance between theelectrochemical potential of the Al matrix and the precipitate freezone. By employing an alloy chemistry that reduces or eliminates thepotential difference between the precipitate free zone and the Almatrix, the present invention increases stress corrosion crackingresistance in one aspect by significantly reducing or eliminating thelocalized corrosion.

The alloy of the present invention may further include up to about 1.0wt. % Silicon, wherein Si may improve castability. Further, lower levelsof Si may be employed to increase strength. For some applications,manganese in amounts up to about 0.3 wt. % may be employed.

The alloy may also contain grain refiners such as titanium diboride,TiB₂ or titanium carbide, TiC and/or anti-grain growth agents such aszirconium, manganese or scandium. If titanium diboride is employed as agrain refiner, the concentration of boron in the alloy may be in a rangefrom 0.0025 wt. % to 0.05 wt. %. Likewise, if titanium carbide isemployed as a grain refiner, the concentration of carbon in the alloymay be in the range from 0.0025 wt. % to 0.05 wt. %. Typical grainrefiners are aluminum alloys containing TiC or TiB₂.

Zirconium, if used to prevent grain growth during solution heattreatment, is generally employed in a range below 0.2 wt. %, preferablyranging from 0.05 wt. % to 0.2 wt. %.

Scandium may also be used in a range below 0.3 wt. %, preferably rangingfrom 0.05 wt. % to 0.3 wt. %.

In another aspect of the present invention, a heat treatment inconjunction with the alloying elements and ratio's optimizesprecipitation at the grain boundaries to increase stress corrosioncracking resistance. Precipitates, such as MgZn₂ and Cu precipitates,form within the metal matrix and at the grain boundary. The precipitatesat the grain boundary are susceptible to corrosive attack. Moreover,precipitation at grain boundary after alloy quench initially includes acontinuous distribution of fine precipitates and disadvantageouslyresults in localized continuous corrosion at the grain boundary.Corrosion of the continuous and fine precipitates at the grain boundary(intergranular corrosion) disadvantageously decreases the alloy's stresscorrosion cracking (SCC) resistance.

The present invention provides a heat treatment to overage the alloy,wherein the heat treatment results in coarsening of the fineprecipitates at the grain boundary to provide a discontinuousdistribution of large precipitates interrupted by Aluminum. Aluminum hasa greater resistance to corrosion than the grain boundary precipitates.Therefore, the discontinuous distribution of large precipitates at thegrain boundary results in a discontinuous mode of corrosion at the grainboundary, which advantageously increases the stress crack corrosionresistance (SCC) of the alloy.

For the purposes of this disclosure the term “overage” or “overagetemper” or “overaged condition” denotes that the time and temperature ofthe heat treatment is selected to sacrifice a degree of strength fromthe alloy's peak strength for improved stress corrosion cracking (SCC)resistance. Applicants state that the term “peak strength” or “peakcondition” denotes the maximum tensile strength or yield strength thatmay be achieved for a given precipitate hardening composition, such as,but not limited to, Al—Zn—Mg—Cu alloy system, wherein the strength isdependent on the temperature and time of the heat treatment.

Preferably, the heat treatment to be used with the Al—Zn—Mg—Cu alloyincluding a Mg and Zn content of 6.0 wt. % or less, and a Cu contentgreater than 0.5 wt. % to provide increased stress corrosion crackingperformance includes at least one treatment at a temperature of greaterthan 340° F., preferably ranging from 340° F. to 380° F., for a timeperiod of 4.0 hours or greater. In one preferred embodiment, the heattreatment includes two stages. In a first stage the casting is heatedfrom room temperature to 250° F. within a time period of one hour andheated from 250° F. to greater than 340° F. within a time period of onehour. In a second stage, the alloy is aged at greater than 340° F. untilachieving an overaged temper, wherein the second stage is conducted forgreater than four hours.

In the overaged temper, the Al—Zn—Mg—Cu alloy system demonstrates 50%higher tensile yield strength than is obtainable from A356.0-T6, whilemaintaining similar elongations and providing stress corrosion cracking(SCC) resistance at a stress of up to 240 MPa and is applicable to partdesigns requiring higher strength than AlSiMg alloys that are readilyavailable today. such as A356.0-T6 or A357.0-T6. Fatigue performance inthe T6 temper is increased over the A356.0-T6 material by 45%.Specifically, high tensile strengths on the order of about 300 MPa andstress corrosion cracking (SCC) resistance at a stress of up to 240 MPaare provided by the Al—Zn—Mg—Cu alloy of the present invention.

In addition to providing increased resistance to stress corrosioncracking (SCC) and providing strengths suitable for automotive castings,the alloy of the present invention provides acceptable general corrosion(pitting) performance. Further, the castability of the inventiveAl—Zn—Mg—Cu alloy is suitable for providing shaped castings.

Although the invention has been described generally above, the followingexamples are provided to further illustrate the present invention anddemonstrate some advantages that arise therefrom. It is not intendedthat the invention be limited to the specific examples disclosed.

Table 1 includes alloy compositions (Alloy composition numbers 1-17)having Mg, Cu, and Zn in accordance with the present invention andincludes the composition of comparative examples. Alloy compositionnumbers 1-5 represent some embodiments of the alloy of the presentinvention having about 3.5 wt. % to about 5.5 wt. %. Zn, about 1.0 wt. %to about 3.0 wt. %. Mg. and about 0.5 wt. % to about 1.2 wt. % Cu,wherein the total Mg and Zn content ranges about from 5.2 wt. % to about5.7 wt. %, the Zn/Mg ratio ranges from about 2.66 wt. % to about 3.75wt. %, and the Cu content ranges from about 0.65 wt. % to about 0.85 wt.%. Alloy composition numbers 6-9 and 18-20 represent comparativeexamples of alloys in which the Cu content is less than 0.5 wt. %. Alloycomposition numbers 10-13 represent comparative examples of alloys inwhich the Mg and Zn content is equal to 6.0 wt. %. Alloy compositionnumbers 14-17 represent comparative examples of alloys in which the Mgand Zn content is greater than 6.0 wt. %. In each of the compositionslisted in Table 1, the Si content is less than 0.05 wt. %, the Fecontent is less than 0.05 wt. %, the Mn content is less than 0.05 wt. %,the Zr content is less than 0.09 wt. %, the B content is less than 0.02wt. %, and the Ti content is less than 0.06 wt. %.

TABLE 1 ALLOY COMPOSITION Alloy Composition Alloy # Zn Mg Cu Mg + ZnZn/Mg 1 4 1.2 0.85 5.2 3.3 2 4 1.5 0.65 5.5 2.7 3 4 1.5 0.85 5.5 2.7 44.5 1.2 0.65 5.7 3.8 5 4.5 1.2 0.85 5.7 3.8 6 4 1.2 0 5.2 3.3 7 4 1.20.35 5.2 2.7 8 4 1.5 0.35 5.5 2.7 9 4.5 1.2 0.35 5.7 3.8 10 4.5 1.5 0.256 3 11 4.5 1.5 0.45 6 3 12 4.5 1.5 0.65 6 3 13 4.5 1.5 0.85 6 2.5 14 4.51.8 0.25 6.3 2.5 15 4.5 1.8 0.45 6.3 2.5 16 4.5 1.8 0.65 6.3 2.5 17 4.51.8 0.85 6.3 2.5 18 4 1.2 0.25 5.2 3.3 19 4 1.5 0 5.5 2.7 20 4.5 1.2 05.7 3.8

Stress Corrosion Cracking Resistance

Tables 2-4 provide the results of boiling salt stress corrosion crackingtesting for test samples having the alloy compositions listed in Table1, wherein the boiling salt stress corrosion cracking testing understress levels of 160 MPa (representative of ˜50% of the alloy's tensileyield strength target) and 240 MPa (representative of ˜75% of thealloy's tensile yield strength) was conducted in accordance with the“Standard Practice for Evaluating Stress-Corrosion Cracking Resistanceof Low Copper 7XXX Series Al—Zn—Mg—Cu Alloys” as described in ASTM G103.In accordance with the guidelines of ASTM G103, stressed specimens aretotally and continuously immersed in boiling solution containing about6% sodium chloride for up to 168 hrs. The specimens are regularlychecked for visual cracking. The time to failure is used to indicate thestress corrosion cracking (SCC) resistance of the aluminum alloys. Atest specimen was considered to have acceptable stress corrosioncracking (SCC) resistance if it could survive the boiling salt test fora time period of 96 hours. The boiling salt stress corrosion cracking(SCC) test was conducted for seven days, wherein test samples that didnot fail during the seven day period were given a value of 168 hours.

Table 2 includes the stress corrosion cracking (SCC) data provided forAl—Zn—Mg—Cu alloys (alloy composition numbers 1-5) having alloyingranges within the scope of the present invention and heat treated to anoveraged condition. The alloy heat treatment included two stages, inwhich the first stage included heating the alloy from room temperatureto 250° F. within one hour. The second stage is aging the alloy to theoveraged condition, wherein Table 2 includes data for aging at 340° F.for 16 hours, and aging at 340° F. for 24 hours.

TABLE 2 BOILING SALT STRESS CORROSION CRACKING TEST 2nd state aging at340 F. for 16 hrs 2nd state aging at 340 F. for 24 hrs Alloy Time(hours) at Time (hours) at Time (hours) at Time (hours) at # 160 Mpa240.0 MPa 60 Mpa 240.0 MPa 1 168 168 168 168 168 168 168 168 168 132 168168 2 168 168 168 107 168 168 168 168 168 132 166 168 3 168 168 168 168168 168 168 168 168 168 168 168 4 168 168 168 166 168 168 168 168 168 58132 168 5 168 168 168 107 168 168 168 168 168 168 168 168

As indicated by Table 2, the Al—Zn—Mg—Cu alloys having alloying rangeswithin the scope of the present invention (Alloys #1-5) and heat treatedto an overaged condition survived at least 96 hours of stress corrosioncracking (SCC) testing in accordance with the boiling salt test.Generally, the test specimens survived from 119 hours to the entirelength of the test (168 hours).

SCC testing was not conducted for test specimens of alloy compositionnumbers 1-9 being treated with a second stage heating step of 340° F.for four hours, since the intergranular corrosion of these testsspecimens as measured using the Standard Practice for EvaluatingIntergranular Corrosion Resistance of Heat Treatable Aluminum Alloys byImmersion in Sodium Chloride+Hydrogen Peroxide Solution in accordancewith ASTM G110 indicated that this period of aging was not suitable toprovide sufficient SCC resistance.

Table 3 includes the stress corrosion cracking (SCC) data provided forAl—Zn—Mg—Cu alloys (alloy composition numbers 6-9) similar to the alloyof the present invention except for having a Cu content of less than 0.5wt. %. Alloy composition numbers 6-9 were tested for stress corrosioncracking (SCC) resistance using the same boiling salt test applied toalloy composition numbers 1-5. Alloy composition numbers 6-9 where agedusing a heat treatment that includes two stages, in which the firststage included heating the alloy from room temperature to 250° F. withinone hour. The second stage is aging the alloy to the overaged condition,wherein Table 2 includes data for aging at 340° F. for 4 hours, andaging at 340° F. for 16 hours.

TABLE 3 BOILING SALT STRESS CORROSION CRACKING TEST FOR AlZnMgCu ALLOYHAVING LESS TI IAN 0.5 wt. % Cu 2nd state aging at 340 F. for 4 hrs 2ndstate aging at 340 F. for 16 hrs Alloy Time (hours) at Time (hours) atTime (hours) at Time (hours) at # 160 Mpa 240.0 MPa 160 Mpa 240.0 MPa 62 2 90 4 4 4 7 4 5 139 4 4 24 44 168 168 18 24 168 8 70 168 168 5 90 125168 168 168 18 44 72 9 18 148 168 4 18 168 18 72 168 4 18 18

As indicated in Table 3, alloy composition numbers 6-9 having a Cucontent of less than 0.5 wt. % displayed a high incidence of failurebefore reaching 96 hours of under stress at 160 MPa or 240 MPa underboiling salt testing. Specifically, only one test specimen having lessthan 0.5 wt. % Cu and aged for 16 hours at 340° F. passed the boilingsalt corrosion test under a stress of 240 MPa, representing ˜75% of thedesired minimum yield strength. Typically, alloy composition numbers 6-9failed within 4-72 hours of testing under boiling salt test.

Table 4 includes the stress corrosion cracking (SCC) data provided forAl—Zn—Mg—Cu alloys (alloy composition numbers 10-14) similar to thealloy of the present invention except for having a combined Zn and Mgcontent of 6.0 wt. % or greater. The alloy heat treatment included twostages, in which the first stage included heating the alloy from roomtemperature to 250° F. within one hour. The second stage includes agingthe alloy to the overaged condition, wherein Table 4 includes data foraging at 340° F. for 4 hours, 340° F. for 16 hours, and aging at 340° F.for 24 hours.

TABLE 4 BOILING SALT STRESS CORROSION CRACKING TEST FOR AlZnMgCu ALLOYHAVING AT LEAST 6.0 wt. % Mg AND Zn. 2nd stage aging 2nd stage aging atAlloy at 340 F. for 4 hrs 340 F. for 16 hrs 2nd stage aging at 340 F.for 24 hrs # 160 MPa 160 MPa 240.0 MPa 160 MPa 240.0 MPa 10 2 2 3 11 3 310 3 5 29 12 20 20 20 168 168 168 24 24 24 168 168 168 21 58 168 13 168168 168 168 168 90 68 90 168 68 114 168 58 76 168 14 1 1 2 15 1 2 3 5 1010 16 20 4 20 60 168 17 44 44 168

As indicated in Table 4, alloy composition numbers 10-17 having a totalMg and Zn content of 6.0 wt. % or greater displayed a high incidence offailure before being subjected to 96 hours of stress at 160 MPa or 240MPa under boiling salt SCC testing. As compared to stress corrosioncracking (SCC) performance of alloy composition numbers 1-5 having atotal Mg and Zn content of less than 6.0 wt. % illustrated in Table 1,alloy composition numbers 10-17 having a total Mg and Zn or 6.0 wt. % orgreater disadvantageously exhibited reduced stress corrosion crackresistance.

Increasing the Mg and Zn content to 6.0 wt. % or greater introduces anexcess of MgZn₂ to the alloy, wherein the excess MgZn₂ decreases thechemical potential at the precipitate free zone (PFZ) relative to thealumina matrix to a level that can not be offset by the incorporation ofCu, without increasing the amount of ALCuFe and AlCuFeSi at the grainboundary, which disadvantageously reduces the alloys fracture toughness.Specifically, Alloy composition numbers 14-17 having a total Mg and Zncontent of 6.3 wt. % exhibited decreased stress corrosion cracking (SCC)resistance than alloy composition numbers 10-13 having a total Mg and Zncontent of 6.0 wt. %.

The data included in Tables 1-4 has been plotted in FIGS. 1 and 2. FIG.1 illustrates the alloy compositions that pass the 96 hour boiling saltwater stress corrosion cracking (SCC) resistance test under a stress of240 MPa (˜75% of the alloy's tensile yield strength target), henceproviding suitable stress corrosion cracking (SCC) resistance. FIG. 2illustrates the alloy compositions that pass the 96 hour boiling saltwater stress corrosion cracking (SCC) resistance test under a stress of160 MPa (˜50% of the alloy's tensile yield strength target), henceproviding suitable stress corrosion cracking (SCC) resistance. Referringto FIGS. 1 and 2, reference lines 10 a, 10 b represents 96 hours ofboiling salt SCC testing, wherein the area 15 a, 15 b under the curvepresented by reference line 10 a, 10 b indicates alloy compositionshaving suitable stress corrosion cracking (SCC) resistance.

To provide sufficient stress crack resistance to pass boiling salt SCCtesting an Al—Zn—Mg—Cu alloy requires that the total Mg and Zn contentbe less than 6.0 wt. % and the Cu content be greater than 0.5 wt. %, andthat the alloy be treated to an overaged condition, preferably includinggreater than four hours aging at 340° F., and even more preferablyincluding 16 hrs of aging at 340° F.

Mechanical Properties

Tables 5-7 provide the results of mechanical testing for test sampleshaving the alloy compositions listed in Table 1, wherein the mechanicalproperties measured included tensile yield strength (TYS), ultimatetensile strength (UTS) and percent elongation (E). Similar to the stresscorrosion cracking (SCC) evaluation, each test sample was treated to atwo-step heat treatment was used, in which the first stage includingkeeping the heat treatment constant at 250° F. for 3 hours. Followingthe first stage, an aging stage was conducted, in which the furnacetemperature was raised to 340° F. for soaking times ranging from 4 to 32hours. The alloy reached peak strength at about 4 hours at 340° F.Overaged conditions were investigated at 16 hours, and 24 hours at 340°F. Test specimens were considered to have acceptable mechanicalproperties when providing tensile yield strength (TYS) on the order ofat least 300 MPa, wherein at the lab scale, test specimens having atensile yield strength (TYS) being on the order of 320 MPa, were highlypreferred.

Table 5 includes the mechanical properties measured for Al—Zn—Mg—Cualloys (alloy composition numbers 1-5) having alloying ranges and ratioswithin the scope of the present invention and heat treated to anoveraged condition.

TABLE 5 Ultimate Yield Tensile 2^(nd) step aging ALLOY Strength StrengthElongation time, hrs # (MPa) (MPa) % @340 F. 1 322.5 377.0 11.0 16.0 1312.0 370.0 11.0 24.0 2 326.0 381.5 12.0 16.0 2 317.0 372.0 12.0 24.0 3325.0 377.5 10.0 16.0 3 328.0 381.5 12.0 24.0 4 295.5 339.5 16.0 4.0 4315.0 368.5 12.0 16.0 4 307.0 362.3 12.0 24.0 5 315.0 372.0 13.5 16.0 5306.0 364.0 14.0 24.0

As indicated by Table 5, the Al—Zn—Mg—Cu alloys having alloying rangeswithin the scope of the present invention (Alloys #1-5) and heat treatedto an overaged condition provided a tensile yield strength (TYS) on theorder of at least 300 MPa. In a preferred embodiment, the Zn/Mg ratio ispreferably less than ˜3.0, since increasing the Zn/Mg ratio for a fixedamount of Zn+Mg to greater than 3.0 typically results in a reduction ofMgZn2 strengthening precipitates disadvantageously reducing tensileyield strength.

For example, alloy composition numbers 1-3 having Zn/Mg rations rangingfrom 2.77 to about 3.0 have a higher tensile yield strength than alloycomposition numbers 4-5 having a Zn/Mg ratio being greater than 3.0. asillustrated in Table 5. The lab scale test specimens having a Zn/Mgratio from 2.77 to 3.0 (alloy composition numbers 1-3) provided atensile yield strength (TYS) being on the order of 320 MPa or greater,whereas test specimen having a Zn/Mg ratio on the order of 3.3 recordedlower tensile yield strength (TYS) values being, in some instances beingcloser to 300 MPa.

As discussed above, the alloy of the present invention includes greaterthan 0.5 wt. % Cu to substantially minimize the effect of the Mg and Znon the difference in corrosion potential between the Al matrix and theprecipitate free zone (PFZ) to provide an alloy having increased SCCresistance, while maintaining tensile properties suitable for highstrength applications. Table 6 illustrates that the increased Cu contentof the Al—Zn—Mg—Cu alloys of the present invention has a minimal effecton the alloy's tensile properties when compared to alloys having lowerCu contents. Specifically, Table 6 includes the tensile propertiesmeasured for Al—Zn—Mg—Cu alloys (alloy composition numbers 6-9 and18-20) having a Cu content of less than 0.5 wt. %.

TABLE 6 Yield Tensile 2nd step aging ALLOY Strength Strength Elongationtime, hrs # (MPa) (MPa) % @340 F. 6 263.5 318.0 16.0 16.0 7 299.5 352.013.0 16.0 8 305.0 354.0 14.0 4.0 8 309.0 361.0 15.0 4.0 8 310.0 362.515.0 16.0 9 311.0 356.0 11.0 4.0 9 304.5 355.0 13.0 16.0 18 304.5 351.014.5 4.0 18 297.0 345.0 14.0 16.0 19 293.5 341.0 16.0 4.0 19 270.0 324.516.0 16.0 20 299.0 342.0 18.0 4.0 20 264.0 316.0 18.0 16.0

As indicated by comparison of Tables 5 and 6, Al—Zn—Mg—Cu alloys havingalloying ranges within the scope of the present invention have similarif not greater tensile properties than similar Al—Zn—Mg—Cu alloys havingless than 0.5 wt. % Cu. For example, alloy composition number 3 having aCu content of 0.85 wt. % provides a tensile yield strength value of 325MPa, while alloy composition number 8 being of similar composition toalloy composition number 1 provides a similar tensile yield strength ofabout 310 MPa when similarly heat treated to an overaged conditionincluding a second stage heat treatment of 340° F. for about 16 hours.The incorporation of Cu within the range of 0.5 wt. % to 1.2 wt. % haslittle to no disadvantageous effect on the tensile yield strength of thealloy, as illustrated by Tables 5 and 6, yet advantageously increasesthe alloy's stress corrosion cracking (SCC) resistance, as illustratedin Tables 2 and 3.

Table 7 includes the mechanical properties measured for Al—Zn—Mg—Cualloys (alloy composition numbers 10-17) having a Zn+Mg content of 6.0or greater.

TABLE 7 Yield Tensile 2nd step aging ALLOY Strength Strength Elongationtime, hrs # (MPa) (MPa) % @340 F. 10 344.6 389.5 14.5 4.0 10 350.0 402.017.0 16.0 11 353.1 399.0 13.0 4.0 11 357.0 407.0 13.0 16.0 12 358.5405.0 13.0 4.0 12 362.3 415.0 13.0 16.0 12 350.5 403.0 13.0 24.0 13370.1 419.8 11.3 4.0 13 366.0 418.0 16.0 16.0 13 354.0 409.3 13.0 24.014 364.5 412.0 13.8 4.0 14 376.0 431.0 14.0 16.0 15 371.3 422.4 11.5 4.016 387.8 434.9 11.0 4.0 17 398.5 445.3 9.5 4.0

Referring to Tables 5 and 7, increasing the Mg+Zn content to 6.0 wt. %or greater provides increases the alloys tensile yield strength, butdisadvantageously decreases the alloy's resistance to stress corrosioncracking (SCC), as indicated in Tables 2 and 4. As explained aboveincreasing the Mg and Zn content in a manner that increases the Zn+Mgcontent to greater than 6.0 wt. % decreases the corrosion potential ofthe precipitate free zone (PFZ) zone relative to the Al matrix to apoint that cannot be offset by the addition of increased Cu withoutproducing an excess of constituent particles of AlFeCu at the grainboundary that decreases the alloys' fatigue resistance and toughness.Further, the increased Zn+Mg also produces higher amounts of MgZn2 atthe grain boundary. which also disadvantageously reduces the alloycomposition's resistance to stress corrosion cracking (SCC).

The tensile properties were also measured from automotive steeringknuckles cast using Vacuum Riserless Casting (VRC)/Pressure RiserlessCasting (PRC) methods and composed of Al—Zn—Mg—Cu aluminum alloys inaccordance with the present invention and having greater than 0.5 wt. %Cu and up to and including 0.9 wt. % Cu. FIG. 3 illustrates a plotdepicting the relationship between the Cu content of the alloy and thetensile yield strength (data line 52), ultimate tensile strength (dataline 51) and elongation of the alloy (data line 50). Each casting wasaged to an overaged condition using a two stage heat treatment includinga first stage at 250° F. for 3 hrs and a second stage at 340° F. for 16hrs. Increases in tensile yield strength 52 and ultimate tensilestrength 51 were recorded in Al—Zn—Mg—Cu alloys with increasing Cucontent from greater than 0.4 wt. % Cu to about 0.9 wt. % Cu.

General Corrosion

General corrosion (corrosion attack mode) was evaluated using ASTM G110corrosion testing, which is the “Standard Practice for EvaluatingCorrosion Resistance of Heat Treatable Aluminum Alloys by Immersion inSodium Chloride+Hydrogen Peroxide Solution”.

Referring to FIGS. 4 a-4 c depicting photographs of tests specimenevaluated using ASTM G110 corrosion testing, wherein alloy compositionnumbers 1, 2, and 4 represent alloy compositions in accordance with thepresent invention, alloy composition numbers 8, 9, 18, and 19, representcomparative alloy compositions having less than 0.5 wt. % Cu, and alloycomposition number 13 represents a comparative alloy composition havinga Zn+Mg content of 6.0 wt. %. The test specimens were cast from adirectionally solidified (DS) mold.

Similar to the evaluations for stress corrosion cracking (SCC)performance and mechanical performance, each test sample was treated toa two-step heat treatment, in which the first stage including keepingthe heat treatment constant at 250° F. for 3 hours. Following the firststage, an aging stage was conducted, in which the furnace temperaturewas raised to 340° F. for soaking times ranging from 4 to 32 hours. Thealloy reached peak strength at ˜4 hours at 340° F. Overaged conditionswere investigated at 16, 24 and hours at 340° F.

In accordance with the procedures detailed in ASTM G110, the testspecimens were immersed in a solution 3.5% NaCl+H₂O₂ for 24 hours. Onceremoved from the corrosive solution, the test specimens wereinvestigated using an optical microscope to determine the mode ofcorrosion attack and depth of corrosive attack.

As depicted in FIGS. 4 a-4 c, test specimen composed of alloycompositions having a Cu content of greater than 0.5 wt. % (alloycomposition numbers 8. 9, 13, and 19) are generally more susceptible tocorrosive attack by pitting than test specimen having less than 0.5 wt.% Cu (alloy composition numbers 8, 9, 18 and 19). More specifically, thefrequency and depth of corrosion sites increases with increasing Cucontent.

The effect of the Cu content on the depth of corrosion furtherillustrated with reference to FIGS. 5 a-5 c. FIGS. 5 a-5 c depict graphsof the depth of corrosion in microns of test specimen evaluated by ASTMG110 corrosion testing, wherein the test specimen include greater than0.5 wt. % Cu (alloy composition numbers 1, 2, and 4) in accordance withthe present invention; and comparative examples having less than 0.5 wt.% Cu (alloy composition numbers 6, 7, 8, 9, and 18). The test specimensincluded castings from directionally solidified (DS) molds. The dataplotted includes the maximum corrosion depth measured and an average ofthe depth for five of the deepest corrosion sites. As indicated by FIGS.5 a-5 c, corrosion depth for alloy compositions having greater than 0.5wt. % Cu is greater than the corrosion depth for alloy compositionshaving less than 0.5 wt. % Cu. Deeper corrosion depth was measured fromcast surfaces, as opposed to machined surfaces, which was believed toresult from microsegregation of Cu on the as cast surface of the DScastings.

Referring to FIGS. 5 a-5 c, the corrosion depth decreased withincreasing aging time. Each of the test samples where aged using a twostage heat treatment, in which the first stage including keeping theheat treatment constant at 250° F. for 3 hours and the second stageincluded raising the furnace temperature to 340° F. for soaking timesranging from 4 or 16 hours. The alloy reached peak strength with asecond stage heat treatment of about 4 hours at 340° F. Overagedconditions were investigated at 16 hours at 340° F. The degree ofoveraging effects the corrosion mode, wherein greater degrees ofoveraging in Al—Zn—Mg—Cu alloys in accordance with the present inventionresult in corrosive attack having a greater degree of pitting, asopposed to intergranular corrosion, and lesser degrees of overaging inAl—Zn—Mg—Cu alloys result in corrosive attack having a greater degree ofintergranular corrosion, as opposed to pitting. Intergranular corrosionresults in a greater corrosion depth than pitting.

Referring to FIGS. 6 a-6 c, the mode of corrosion was evaluated bysectioning the test samples and visually inspecting the samples crosssection with an optical microscope. Referring to alloy composition #18having a Cu content on the order of 0.25 wt. % in FIGS. 6 a and 6 b,when heat treated to peak strength conditions, i.e. a first stage heattreatment at 250° F. for 3 hours followed by a 4 hour second stage heattreatment at 340° F., the corrosion mode of the alloy composition may becharacterized as pitting. Referring to FIG. 6 a, corrosion depthgenerally increases in cast surfaces of alloy compositions including 0.5wt. % or greater Cu contents greater, such as alloy composition numbers1 and 13 composed of 0.85 wt. % Cu, when compared to alloy compositionshaving less than 0.5 wt. % Cu, such as alloy composition number 18having 0.25 wt. % Cu. Referring to FIGS. 6 b and 6 c, increasing the Cucontent to 0.35 wt. % or 0.45 wt. %, such as alloy composition numbers7, 8, 9 and 11, changes the mode of corrosion to at least partiallybeing intergranular corrosion. The mode of corrosion may be entirelyintergranular corrosion in Al—Zn—Mg—Cu compositions aged to peakstrength (i.e. 2^(nd) step at 340° F. for 4 hours) and having greaterthan 0.5 wt. % Cu, such as alloy composition number 16 having a Cucontent of 0.65 wt. %, as depicted in FIG. 6 c.

FIG. 6 c further illustrates at least one advantage of the presentinvention, in which a heat treatment is provided to overage the alloyresulting in coarsening of the fine precipitates at the grain boundaryto provide a discontinuous distribution of large precipitatesinterrupted by Aluminum. As discussed above, Aluminum has a greaterresistance to corrosion than the grain boundary precipitates. Therefore,the discontinuous distribution of large precipitates at the grainboundary results in a discontinuous mode of corrosion at the grainboundary, which advantageously increases the stress crack corrosionresistance (SCC) of the alloy.

The heat treatment to provide the overaged condition included twostages, in which the first stage included heating the alloy from roomtemperature to 250° F. within one hour. The second stage is aging thealloy to the overaged condition, wherein Table 4 includes data for agingat 340° F. for 4 hours (peak condition), 340° F. for 16 hours (overagedcondition), and aging at 340° F. for 24 hours (overaged condition).Alloy composition #16, as depicted in FIG. 6 c, clearly illustrates thatthe heat treatment of the present invention, i.e. overaging for greaterthan four hours at 340° F. during the second stage of the heattreatment, advantageously converts the mode of corrosion fromintergranular to pitting.

Castability

The castability of the Al—Zn—Mg—Cu alloy of the present invention havinggreater than 0.5 wt. % Cu was assessed using pencil probe for hotcracking index and spiral molds for fluidity. The hot cracking index isthe smallest diameter of the central connection rod on the pencil probecasting that does not exhibit cracking, wherein the lower the value forthe hot cracking index the better the hot cracking resistance of thealloy composition.

FIG. 7 illustrates the effect of the Cu content on the hot crackingindex of an Al—Zn—Mg—Cu alloy, having 4.5 wt. % Zn, 0.09 wt. % Zr. and1.8 wt. % Mg or 1.5 wt. % Mg. The Cu content hot cracking resistance ofthe Al—Zn—Mg—Cu alloy of the present invention is not substantiallyaffected by the incorporation of Cu being greater than 0.5 wt. %.

FIG. 8 illustrates the effect of the Cu content on the fluidity of anAl—Zn—Mg—Cu alloy having 4.5 wt. % Zn, 0.09 wt. % Zr, and 1.8 wt. % Mgor 1.5 wt. % Mg. Cu had no appreciable effect on fluidity in Al—Zn—Mg—Cucompositions including 1.8 wt. % Mg, and provides a slight increase influidity in Al—Zn—Mg—Cu compositions including 1.5 wt. % fluidity.

While the present invention has been particularly shown and describedwith respect to the preferred embodiments thereof, it will be understoodby those skilled in the art that the foregoing and other changes informs of details may be made without departing form the spirit and scopeof the present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

We claim:
 1. A cast aluminum part comprising an Al—Zn—Mg—Cu alloy,wherein: the alloy comprises about 4.0 wt. % to about 4.5 wt. % Zn;about 1.2 wt. % to about 1.8 wt. % Mg; about 0.6 wt. % to about 0.85 wt.% Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; lessthan about 0.30 wt. % Fe; a total Mg and Zn content of less than about6%; and, incidental impurities; and, a cast aluminum part is producedfrom a process comprising producing a melt comprising the alloy having afluidity which exceeds a length of 7 cm in a spiral mold for the castingprocess; casting at least a portion of the melt into a mold to providethe cast aluminum part; and, heat treating the cast aluminum part to anoveraged condition, the process including a T6 heat treatment and agingat a temperature greater than about 340° F. for greater than about fourhours; wherein, the cast aluminum part has a time-to-failure of greaterthan 96 hours under ASTM G103 testing conditions for stress corrosioncracking.
 2. The cast aluminum part of claim 1, wherein heat treatingthe cast aluminum part to the overaged condition further comprises:heating the cast aluminum part from about room temperature to atemperature in a range of about 200° F. to about 300° F. within a timeperiod of one hour.
 3. The cast aluminum part of claim 1, wherein thealuminum alloy melt comprises about 0.65 wt. % to about 0.85 wt. % Cu.,and the aging of the cast aluminum part occurs at a temperature rangingfrom about 340° F. to about 380° F. for greater than about four hours 4.The cast aluminum part of claim 1, wherein the time-to-failure underASTM G103 testing conditions is greater than 96 hours.
 5. The castaluminum part of claim 1, wherein the magnesium concentration rangesfrom a concentration of about 1.5 wt. % to 1.8 wt. %, and the ratio ofZn to Mg is less than about 3.0.
 6. The cast aluminum part of claim 1,wherein the Cu concentration ranges from 0.65 wt % to 0.85 wt % and theratio of Zn to Mg ranges from about 2.7 to about 3.8.
 7. The castaluminum part of claim 1, wherein the copper concentration ranges from0.65 wt % to 0.85 wt % and the ratio of Zn to Mg ranges is about 2.7,about 3.3, or about 3.8.
 8. The cast aluminum part of claim 1, whereinthe Cu concentration ranges from 0.65 wt % to 0.85 wt % and total Mg andZn content ranges from 5.2 wt % to 5.7 wt %.
 9. A method of making acast aluminum part comprising: creating an aluminum alloy meltcomprising about 4.0 wt. % to about 4.5 wt. % Zn; about 1.2 wt. % toabout 1.8 wt. % Mg; greater than about 0.5 wt. % to about 0.85 wt. % Cu;less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less thanabout 0.30 wt. % Fe; a total Mg and Zn content of less than about 6%;and incidental impurities; casting at least a portion of the melt into amold to provide a cast aluminum part; and, heat treating the castaluminum part to an overaged condition, the heat treating including a T6heat treatment and aging at a temperature greater than about 340° F. forgreater than about four hours; wherein, the melt has a fluidity whichexceeds a length of 7 cm in a spiral mold for castability; and, the castaluminum part produced by the method has an elongation of 8% or greater;and, a time-to-failure of greater than 96 hours under ASTM G103 testingconditions for stress corrosion cracking.
 10. The method of claim 9,wherein heat treating the cast aluminum part to the overaged conditionfurther comprises: heating the cast aluminum part from about roomtemperature to a temperature in a range of about 200° F. to about 300°F. within a time period of one hour.
 11. The method of claim 9, whereinthe aluminum alloy melt comprises about 0.65 wt. % to about 0.85 wt. %Cu., and the aging of the cast aluminum part occurs at a temperatureranging from about 340° F. to about 380° F. for greater than about fourhours.
 12. The method of claim 9, wherein the ratio of Zn to Mg rangesfrom about 2.66 to about 3.75.
 13. An Al—Zn—Mg—Cu aluminum alloy,comprising: about 4.0 wt. % to about 4.5 wt. % Zn; about 1.2 wt. % toabout 1.8 wt. % Mg; greater than about 0.5 wt. % to about 0.85 wt. % Cu;less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less thanabout 0.30 wt. % Fe; a total Mg and Zn content of less than about 6%;and, incidental impurities; wherein, the alloy has a fluidity whichexceeds a length of 7 cm in a spiral mold for a shaped casting process;and, when used in a casting process that includes T6 heat treatment andaging at a temperature greater than about 340° F. for greater than aboutfour hours, provides a shaped aluminum casting having (i) an elongationof 8% or greater and (ii) a time-to-failure of greater than 96 hoursunder ASTM G103 testing conditions for stress corrosion cracking. 14.The aluminum alloy of claim 13, wherein the alloy comprises about 0.65wt. % to about 0.85 wt. % Cu.
 15. The aluminum alloy of claim 13 furthercomprising at least one grain refiner selected from a group consistingof boron, carbon and combinations thereof.
 16. The aluminum alloy ofclaim 1 further comprising at least one anti-grain growth agent selectedfrom the group consisting of zirconium, scandium, manganese andcombinations thereof.
 17. The aluminum alloy of claim 1, wherein themagnesium concentration ranges from a concentration of about 1.5 wt. %to 1.8 wt. %, and the ratio of Zn to Mg is less than about 3.0.
 18. Thealuminum alloy of claim 1, wherein said magnesium is at a concentrationof about 1.5 wt. % to 1.8 wt. %.
 19. The aluminum alloy of claim 1,wherein the ratio of Zn to Mg is less than about 3.0.
 20. The aluminumalloy of claim 11, wherein said copper is at a concentration of about0.7 wt. % to 0.8 wt. %.
 21. The aluminum alloy of claim 1, wherein theratio of Zn to Mg ranges from about 2.77 to about 3.0.
 22. The aluminumalloy of claim 1, wherein the concentration of copper is selected fromthe group consisting of about 0.6 wt. %, about 0.7 wt. %, and about 0.8wt. %.
 23. The aluminum alloy of claim 1, wherein the ratio of Zn to Mgranges from about 2.66 to about 3.75.